Injection devices, typically in the form of pen shaped syringes, are especially used by diabetics who have to inject themselves frequently with an insulin preparation to keep their blood glucose level within tolerable limits. However, recently different kinds of medicine, for example growth hormone, have been administered in the same fashion.
The doses are mainly set by rotating, or dialing, part of the pen syringe relatively to the rest of the syringe and numbers forming a scale on the rotatable part of the syringe are moved in relation to an indicating mark on the rest of the syringe to indicate the set dose. When the user sets the dose, the syringe is actuated in order to express the dose of medicine. The actuation is normally provided by the user pressing some kind of button by a single movement up to a stop. This makes the injection stage independent of any need to assess what dose is being injected.
The injection device should be as simple as possible to use, i.e. the normal use should only imply setting a dose and injecting the set dose, and both these steps should be simple to perform and this condition is met by most prior-art pen devices.
However, for some users the actual step of expression the dose causes anxiety and a device which automatically expressed the dose on demand would be desirable. Such a facility also could provide for a more repeatable and smooth injection of the fluid. According to U.S. Pat. No. 5,104,380 this has been achieved by a syringe device comprising a body and a rotatable dose setting device mounted on the body and capable of being moved to a selected set position, a latch arranged to retain the setting device in the set position, and means arranged to release the latch to cause the set dose to be expelled. Movement of the dose setting device to the selected set position is accompanied by rotational straining of a spring, which, when the latch is released, provides the force for expelling the set dose. When the latch is released, the setting device is returned to an original position to drive a plunger through a one-way clutch to expel the set dose. The disclosed driving means comprises a quick pitch screw thread arrangement for transforming rotation of the setting device into linear movement of the plunger. The body is adapted for receiving a cartridge containing a fluid to be injected by having a cartridge container removable from the body for insertion of a cartridge and then removal of the cartridge container is arranged to release the quick pitch screw thread device thus allowing the plunger to be returned to an initial position.
However, not all pen syringes offer the opportunity to cancel a set dose, so if a dose once set is not wanted for injection the only way to bring the syringe back in its neutral position is to spill the dose. With syringes by which large doses may be set or in case the medicine is very expensive, as is the case with growth hormone, this is not acceptable.
In order to solve this problem U.S. Pat. No. 5,626,566 discloses a pen shaped syringe for repetitive injection of individually set doses of a medicine from a cylinder ampoule reservoir, comprising a dose setting member which allow a dose set to be cancelled by incorporating means provided to release a unidirectional coupling between a piston drive member and the dosing member. However, this design both requires a release mechanism to be actuated when a dose is to be reset as well as a manual actuating mechanism when the medicine is to be expressed.